Light, temperature, and pH control of aqueous azopyridine-terminated poly(N-isopropylacrylamide) solutions

Abstract

Azopyridines (AzPy) act as light-sensitive groups that undergo reversible cis–trans isomerization upon UV irradiation, as hydrogen-bond acceptors, and as ionizable moieties. The kinetics of the thermal cis- to trans-AzPy deactivation are slow except when the Py nitrogen is H-bonded or cationic. The properties of AzPy were used here to control the phase transition of aqueous solutions of α-azopyridine-ω-n-dodecyl-poly(N-isopropylacrylamides) (C12-PN-AzPy) with the molar mass (M_n) ranging from 5800 to 19 700 g mol⁻¹. The C12-PN-AzPy polymers form cationic star-micelles in solutions of pH 3 and flower-micelles in neutral and basic solutions. This diversity of micelle morphology underlies the temperature-, pH- and UV-irradiation-driven phase transition of aqueous C12-PN-AzPy solutions as demonstrated by turbidimetry, ¹H NMR spectroscopy, and microcalorimetry. Unlike azobenzene, the commonly used photoresponsive moiety to actuate amphiphilic polymers, AzPy can affect the thermoresponsive behavior of polymers in response to three orthogonal triggers: pH, through changes in ionization; light, via trans–cis photoisomerization; and time, from hours to a few ms, via the kinetics of the dark cis–trans relaxation. The study leads the way to responsive sensors or actuators in the form of aqueous fluids, hydrogels, or films by the application of light and changes of temperature and pH in permutable sequences.